Disposable absorptive devices are well known. Numerous disposable products in the form of diapers, incontinent devices, sanitary napkins, and panty liners are readily available to the consumer. The function of all of these devices is to absorb and retain body fluids. The backsheet used to cover these products is typically a flexible plastic film which is impermeable to liquids and vapors. The function of the backsheet is to contain body fluids within the absorbent material and to prevent liquid passage which could cause soiling of garments worn by the user or other surroundings such as bedding. A major drawback of the conventional liquid impermeable backsheet is that it is also vapor impermeable. The disadvantage of a vapor impermeable backsheet is that body fluids in the absorptive device are unable to evaporate thus creating a hot saturated absorbent mass. Since this hot saturated absorbent mass is held against the user's body, an unpleasant and uncomfortable feeling to the user is unavoidable. It is also believed that rashes and irritations are more likely among users of disposable absorptive devices which incorporate a vapor impermeable backsheet.
Prior workers have attempted various methods to facilitate removal of vapors in disposable absorptive devices. U.S. Pat. No. 3,989,867 discloses an absorptive device having a backsheet with bosses and small apertures at the apex of the bosses. The apertures are uniformly distributed and take up 1/2% to 10% of the available permeation area of the backsheet to allow vapor transmission while preventing liquid passage at typical pressures encountered in use. U.S. Pat. No. 4,059,114 relates to a disposable shield for garment protection and everyday feminine hygiene with a fluid barrier in the form of a soft, pliable, rattle-free, moisture vapor permeable layer which is preferably a liquid-impermeable layer of a blown microfiber web. European Patent Application No. 83305609.0 relates to a breathable panty liner with one or more plies of a fibrous, liquid repellent, air permeable layer. Said layer is comprised of treated cellulosic fibers or synthetic polymeric fibers of polyethylene, polypropylene, or polyester. None of these prior art devices utilize a microporous film to impart vapor-transmissive characteristics to the disposable absorptive device.
Microporous films or membranes have a structure that enables vapors to pass through them. The effective pore size is at least several times the mean free path of the flowing molecules, namely from several micrometers down to about 100 Angstroms. Such sheets are generally opaque, even when made of a transparent material, because the surfaces and the internal structure scatter visible light.
Microporous membranes or films have been utilized in a wide variety of applications such as for the filtration of solids, diffusion barriers or separators in electrochemical cells, and in cloth laminates for use as raincoats and other outer wear. Microporous membranes or films are also utilized to make surgical dressings, bandages, and in other fluid transmissive medical applications.
The art of preparing microporous films or membranes is not restricted but rather is replete with a wide variety of methods for producing such articles.
U.K. Patent Application GB No. 2,026,381 A discloses the preparation of membranes having a porous surface by blending polymer with a liquid component to form a binary two-phase system which, in the liquid aggregate state, has a region of miscibility and a region with miscibility gaps, forming a sheet of the blend, casting the film into a bath of the liquid component and removing the liquid component to provide porosity. The resultant non-oriented porous sheet has a relatively low tensile strength.
U.S. Pat. Nos. 3,953,566, 3,962,153, 4,096,227, 4,110,392, 4,187,390 and 4,194,041, describe the preparation of porous articles, including microporous sheets, formed exclusively of polytetrafluoroethylene (PTFE), a non-thermoplastic polymer, which are characterized by having polymer nodes connected by fibrils. Such articles are produced by extruding a paste comprised of PTFE particles and a lubricant, removing the lubricant and stretching and annealing the resultant product. The resultant product is a sintered, oriented porous film of PTFE.
U.S. Pat. Nos. 4,100,238 and 4,197,148 describe the preparation of microporous films by extruding a two component blend, solvent leaching one dispersed component and stretching the resultant leached film to obtain a desired porosity. The blend consists of polymer and a leachable, non-miscible substance. Once the leachable dispersed polymer phase is removed and the film oriented, a porous film results.
U.S. Pat. No. 3,679,540 discloses a method for making a microporous polymer film by cold stretching an elastic polymer film until porous surface regions are formed by film failure, hot stretching the cold stretched film until fibrils and pores or open cells are formed and then heat setting the resultant film. Controlled porosity is generally not attained in such films because they do not always uniformly fibrillate to a specific pore size.
Certain U.S. patents disclose the preparation of porous polymer films by blending into the polymer non-miscible leachable particulate substance such as starch, salts, etc., forming a sheet and leaching the particulate substance from the polymer sheet. Such U.S. Patents include U.S. Pat. Nos. 3,214,501 and 3,640,829. U.S. Pat. No. 3,870,593 discloses the preparation of a microporous polymer sheet by blending non-miscible, non-leachable filler into the polymer, forming a sheet of the blend and stretching the sheet to form pores which are initiated at the sites of the filler particles.
U.S. Pat. No. 4,539,256, which patent is hereby incorporated by reference, teaches a method of making a microporous sheet which comprises the steps of melt blending a crystallizable thermoplastic polymer with a compound which is miscible with the thermoplastic polymer at the polymer melting temperature but immiscible on cooling below the polymer melting temperature, forming a sheet of the melt blend, cooling the sheet to a temperature at which the compound becomes immiscible with the polymer to cause phase separation between the thermoplastic polymer and the compound to provide a sheet comprising a first phase comprised of particles of thermoplastic polymer in a second phase of said compound, orienting the film in at least one direction to provide a network of interconnected micropores throughout the sheet. The microporous sheet comprises about 30 to 80 parts by weight crystallizable thermoplastic polymer and correspondingly about 70 to 20 parts by weight of the compound. The oriented sheet has a microporous structure characterized by a multiplicity of spaced randomly dispersed, equiaxed, non-uniform shaped, non-porous particles of the thermoplastic polymer which are coated with the compound. Adjacent thermoplastic particles within the sheet are connected to each other by a plurality of fibrils consisting of the thermoplastic polymer. The fibrils radiate in three dimensions from each particle. The compound may be removed from the sheet, e.g., by solvent extraction.
The use of porous films as backsheets for disposable diapers is, of course, known. U.S. Pat. No. 4,347,844 relates to the preparation of a porous sheet by blending into the polymer a particulate substance, forming the sheet, breaking the particles within the sheet under a compressive force, and utilizing said sheet as a backsheet for a disposable diaper. U.S. Pat. No. 4,364,985 relates to the preparation of a porous sheet by blending into the polymer a particulate substance, forming a sheet, abrading or buffing the surface of the sheet, and using the resulting sheet as a backsheet for a disposable diaper. A disadvantage of these two porous backsheets is their relatively poor compatibility with commercially available closure tapes due to their poor physical strength.
Another problem with known microporous backsheets is their lack of user identification as a vapor permeable backsheet. Since microporous film is generally opaque white due to light scattering in the internal structure, microporous backsheets appear identical to the conventional vapor impermeable plastic backsheets.